1. Field of the Invention
This invention relates generally to microwave heating apparatus and more particularly to a solid state microwave power source for microwave heating apparatus such as a microwave oven.
2. Description of the Prior Art
Microwave heating apparatus and more particularly the microwave oven is an outgrowth of the resistance heated electric oven and currently uses a low cost magnetron. Instead of electric power being used to heat the food by thermal conduction, microwave energy is introduced into the oven where it is absorbed by the water molecules within the food. The big difference from the resistance heated oven is that energy is efficiently absorbed by the food and the heating takes place within the bulk of the food rather than at the surface. The net result is that food is heated much more rapidly and most of the power is used to heat the food and very little is lost heating the oven and surroundings.
The operating frequency of a domestic microwave oven is commonly 2450 MHz, although some other frequencies are allowed. In North and South America, a frequency of 915 MHz is also allowed for industrial heating applications. The choice of operating frequency is normally based on the convenience of the magnetron. By choosing the 2450 MHz range, a relatively small magnetron tube can be used as the volume and mass of the magnetron is inversely proportional to the third power of the frequency. If, for example, a 915 MHz frequency is chosen, the magnetron and waveguide feed is typically larger and more expensive and is favored for industrial heating applications.
Since the invention of the transistor in 1947, there has been a steady substitution of the vacuum tube electronics by solid state devices. As a result, solid state microwave ovens were being patented as far back as 1971. An example comprises U.S. Pat. No. 3,557,330, entitled, "Solid State Microwave Oven", issued to Bruce R. McAvoy of the Westinghouse Electric Corporation, the assignee of the present invention. Another example of such apparatus is shown and disclosed in U.S. Pat. No. 3,691,338, entitled, "Solid State Microwave Heating Apparatus", issued to K Chang on Sep. 12, 1972. The combination of both magnetron and solid state type heating apparatus is further shown and described in U.S. Pat. No. 3,867,607, entitled, "Hybrid Microwave Heating Apparatus", issued to T. Ohtani on Feb. 18, 1975.
In early versions of the microwave oven, the food tended to be unevenly cooked. This was due to the presence of standing electromagnetic waves within the oven. Later ovens incorporated a small motor driven paddle to "stir" the microwave energy as it entered the oven and/or incorporated a rotating carousel within the oven onto which the food was placed.
More recently, a family of microwave silicon bipolar transistors for radar systems have been developed by the Westinghouse Electric Corporation, the present assignee. However, the cost of these devices has heretofore made it prohibitive for applications involving microwave heating because of the packaging and matching circuitry associated therewith and because relatively low cost magnetrons are readily available.